Typically, while drilling an oil or gas well, the density of the drilling mud must be controlled so that its hydrostatic pressure is not less than the pore fluid pressure in any formation along the uncased borehole (where the borehole is open to the formations). Otherwise, formation fluid may flow into the wellbore, and cause a "kick". Kicks can lead to "blowouts" if the flow is not stopped before the formation fluid reaches the top of the well. And if the fluid contains hydrocarbons, a spark can escalate a blowout into an inferno. Kicks result where the mud weight is too low to balance formation pore fluid pressures. Excessive overbalance, where the hydrostatic pressure of the drilling mud greatly exceeds the pore fluid pressure, is also undesirable. It can induce fractures in the borehole wall that causes lost returns- (i.e., loss of drilling fluid). The drill pipe may also get stuck along contact zones with the borehole wall if the hydrostatic pressure of the mud is too much in excess of the pore fluid pressure. Furthermore, overbalanced mud reduces the penetration rate of the drill bit. This reduced penetration rate increases drilling time which directly increases the associated drilling costs. Therefore, to optimize drilling performance and minimize drilling problems, the mud weight must be adjusted according to the varying pore fluid pressures along the wellbore.
Seismic interval velocities are used to develop pre-drill pore fluid pressure predictions for oil and gas wells. The reliability of these predictions impacts drilling performance, and the risk of kicks, lost returns, and stuck pipe. Pore fluid pressures are also computed from wireline sonic logs as part of the follow-up analysis for a completed well. Post-drill calculations are used to verify the accuracy of the pre-drill forecasts, and to aid in making pore fluid pressure predictions for future wells in the vicinity.
When the pore fluid pressure exceeds the pore fluid's hydrostatic value, the fluid is "overpressured" or "abnormally pressured." It has long been recognized that a formation's sonic velocity is sensitive to the amount of overpressure. However, the prior art has not accounted for varying causes of overpressure and its effects on sonic velocity. Failure to do so can lead to significant errors in the pore fluid pressure predictions.
Under "normal" pressure conditions, a formation is in hydraulic communication with the surface and the pore fluid pressure equals the pore fluid's hydrostatic value. As burial over geologic time increases the overburden stress, the pore fluid is easily expelled out of pore spaces to accommodate sediment compaction. It is only the portion of the overburden stress carried by the sediment grains that causes the grains to compact. This stress, hereafter referred to as the "effective stress," is equal to the difference between the overburden stress and the pore fluid pressure. For normal pressure, the effective stress, density, and sonic velocity all increase with depth.
Overpressure most commonly occurs when low permeability inhibits pore fluid from escaping as rapidly as the pore space would like to compact. Excess pressure develops as the weight of newly deposited sediments squeezes the trapped fluid. Because the fluid has a low compressibility, it supports a majority of the additional overburden load, and retards further compaction. As a result, the effective stress and sonic velocity change more slowly during subsequent burial than they would under normal pressure conditions. On a plot of sonic velocity versus depth along a well, the onset of overpressured formations coincides with the depth at which the velocities start to fall below the trend line followed by normally pressured formations. This overpressuring process is referred to as "undercompaction" or "compaction disequilibrium."
While undercompaction is the most common cause of overpressure, it is not the only cause. Abnormally high pressure can also be generated by thermal expansion of the pore fluid ("aquathermal pressuring"), hydrocarbon maturation, charging from other zones, and expulsion/expansion of intergranular water during clay diagenesis. With these mechanisms, overpressure results from the rock matrix constraining expansion of the pore fluid.
Unlike undercompaction, fluid expansion can cause the pore fluid pressure to increase at a faster rate than the overburden stress. When this occurs, the effective stress decreases as burial continues. The formation is said to be "unloading." Since sonic velocity is a function of the effective stress, the velocity also decreases and a "velocity reversal zone" develops. A velocity reversal zone is an interval on a graph depicting sonic velocity as a function of depth along a well in which the velocities are all less than the value at some shallower depth.
A large portion of the porosity loss that occurs during compaction is permanent; it remains "locked in" even when the effective stress is reduced by fluid expansion. A formation that has experienced a greater effective stress than its current value will be more compacted and have a higher velocity than a formation that has not. Consequently, the relationship between sonic velocity and effective stress is no longer unique when unloading occurs. In other words, for every effective stress, there is no longer one unique sonic velocity. The sonic velocity follows a different, faster velocity-effective stress relationship during unloading than it did when the effective stress was building. This lack of uniqueness is called "hysteresis." Since fluid expansion causes unloading, while undercompaction does not, hysteresis effects make the sonic velocity less responsive to overpressure generated by fluid expansion. As a result, the pore fluid pressure corresponding to a given sonic velocity at a given depth within a velocity reversal zone can be significantly greater if the overpressure was caused by fluid expansion rather than undercompaction. Therefore, the sonic velocity of an overpressured formation is indirectly dependent upon both the magnitude and the cause of overpressure.
While velocity reversal zones are indicative of formations that have undergone unloading, not all velocity reversals are the result of unloading. The velocity will also drop across a transition from a normally pressured sand/shale sequence to a massive, undercompacted shale. The cause of a reversal can be determined by comparing velocity-effective stress data from inside and outside the velocity reversal zone. If the velocity reversal zone data track a separate, faster trend, then the formations within the velocity reversal zone have undergone unloading. If all of the data follow the same trend, then no unloading has occurred.
There are numerous methods for estimating pore fluid pressure from velocity data. The simplest approach utilizes empirical overlays that relate sonic velocity deviations from the "normal trend" (the value expected for hydrostatic pressure) to the pore fluid pressure gradient. One drawback with this method is that it requires a normal trend line for reference. The normal trend is obtained by extrapolating a curve drawn through data assumed to be in hydrostatically pressured zones. However, in some areas, overpressure can start almost from the surface, so there may be little or no data with which to estimate the normal trend.
To work consistently, the overlay approach also requires separate overlays for undercompaction and unloading zones. An overlay constructed from undercompaction data will underestimate the pore fluid pressure in an unloading zone, and vice versa. In some areas this is not a problem, because all of the overpressure is caused by the same type of mechanism. For instance, along the Texas/Louisiana Gulf Coast, the onset of overpressured formations coincides with the start of a velocity reversal zone. However, often there are not enough data to construct local overlays. When overlays from another area with a different source of overpressure are used, significant errors can result.
Other methods for estimating pore fluid pressure make use of the fact that the sonic velocity actually depends upon the effective stress. They equate pore fluid pressure to the difference between the overburden stress and the effective stress. The "equivalent depth" method does this by setting the effective stress in an overpressured zone equal to that computed at another depth where the sonic velocity is the same and the pore fluid pressure is estimated to be hydrostatic. Other techniques use a velocity-effective stress relation constructed from available well data.
All current effective stress approaches fail to recognize that a single velocity-effective stress relation is not always sufficient. Hysteresis effects must be accounted for when overpressure causes unloading. One relationship applies when the current effective stress is the highest ever experienced by the formation. This includes normal pressure and overpressure caused by undercompaction. A second relationship is needed when the effective stress has been reduced, which would be the case when fluid expansion is an important overpressure source. The present invention provides a method for computing pore fluid pressures that accounts for hysteresis effects in the relationship between sonic velocity and effective stress.